1. Field of the Invention
The present invention relates to a current collector for SOFC fuel-cell piles.
2. Background Information
A fuel-cell pile is provided with a plurality of fuel cells as essential components. A fuel cell in turn comprises a cathode, an electrolyte and an anode. An oxidizing agent such as air is supplied to the cathode and a fuel such as hydrogen to the anode. Both fuel and oxidizing agent are referred to in general as process materials hereinafter.
Various types of fuel cells exist. An example is the SOFC fuel cell, which is also known as the high-temperature fuel cell, since its operating temperature can be as high as 1000.degree. C.
At the cathode of a high-temperature fuel cell, oxygen ions are formed in the presence of the oxidizing agent. The oxygen ions pass through the electrolyte to the anode side, where they recombine with hydrogen from the fuel to form water. The recombination reaction releases electrons and thereby generates electrical energy.
A SOFC fuel cell contains a solid electrolyte, which conducts the O.sup.2- ions but not electrons. Yttria-stabilized zirconia (YSZ) is usually used as material for the solid electrolyte.
Large powers are achieved by stacking a plurality of fuel cells together and connecting them electrically in series. The element which connects two fuel cells is known as an interconnector. It provides both electrical and mechanical coupling of two fuel cells. The connecting element is also used to form the cathode or anode chambers. A cathode chamber contains a cathode and an anode chamber. Such stacked fuel cells are known as fuel-cell piles.
From the prior art there is known an interconnector made from ceramic material, for example lanthanum chromite (LaCrO.sub.3). This interconnector indeed exhibits suitable electrical conductivity at high temperatures and can also be readily matched to the thermal expansion behavior of the cell material of the fuel-cell pile. The ceramic material is very expensive, however, in addition to which production of interconnectors therefrom is a complex process. Thereby high manufacturing costs are also incurred.
Another interconnector known from the prior art is made from a metallic material. For this purpose a heat-resistant ferritic alloy such as Cr5Fe1Y.sub.2 O.sub.3 is preferably used. Cr5Fe1Y.sub.2 O.sub.3 is a mechanical alloyed alloy which is 99% metallic and is a powder metallurgical ("PM") alloy. Cr5Fe1Y.sub.2 O.sub.3 is produced using powders, high-energy milling and sintering. Cr5Fe1Y.sub.2 O.sub.3 contains 94% chromium, 5% iron and 1% Y.sub.2 O.sub.3. Cr5Fe1Y.sub.2 O.sub.3 serves to improve certain properties, such as fatigue and corrosion resistance. Because of the high operating temperatures in combination with high O.sub.2 partial pressure on the cathode side, an oxide layer is formed on the metallic interconnector material.
This oxide layer must now satisfy stringent requirements of high temperature stability and conductivity. In the prior art, these requirements are met only by chromium oxide layers. These in turn suffer from the disadvantage, however, that the cathode in particular becomes damaged by volatilization of chromium oxides under the given high-temperature operating conditions. For this reason, the prior art provides for coating the cathode side with a special full-surface barrier layer, of LaCrO.sub.3, for example, in order to prevent volatilization of the chromium oxides.
It is also known from the prior art that, by addition of aluminum, the alloy forms a cover layer of Al.sub.2 O.sub.3. This cover layer in turn is more stable than the chromium oxide cover layer, but the cover layer of Al.sub.2 O.sub.3 has only vanishingly low electrical conductivity.